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How does one mill a 60" radius without a really huge rotary table?

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  • #31
    Long radii are actually a lot easier and a whole lot faster if you don't have CNC.
    Analog is the way to go for a wonderfully smooth curve instead of a thousand tiny little steps.
    You need a manual milling machine, a fly cutter and small rotary turntable.
    Below is a photo of a 200 inch radius of curvature polishing tool that we made.
    Below that is a reference showing how mirror makers produce long radius curves.
    After 15 minutes of setup, it took about 10 minutes to rotate the turntable and produce the curve.
    The turntable is tilted the amount of the Sag or calculated as shown below.
    If tilted the other direction, a concave curve will be made.
    If you only have a small fly cutter, the curve can be generated in two passes, but the angle will have to be recalculated.
    Hope this helps,


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    • #32
      Unfortunately that technique produces a part curved in two planes. and producing a parallel back side is a real bitch!

      https://www.liutaiomottola.com/formulae/sag.htm

      Sag calculator

      Lots of material to be removed with a fly cutter! ;-)
      Last edited by CalM; 09-15-2020, 12:18 AM.

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      • #33
        I don't think some here are readings the posts
        As I said
        " This setup makes errors almost impossible as the link could have a turnbuckle to make arc length adjustments
        Skilled machinists would use depth mikes , exact chord length and balls (with trig) to determine accuracy of the arc.

        I have seen a 18 foot Diameter Hydro-Electric Generator plate being machined on a Bridgeport, and NO, the weight was not a table load .
        The pivot point in the center had a large bearing for that .
        If you work in a Job Shop, "ALL" things are possible ----I think so many here are ingrained that only CNC can do accurate work..NO, CNC can do repetitive work !
        If it isn't set up right, it will makes errors too, only faster !
        Keeping an open mind to all tools and things makes impossible tasks possible.
        The poster did not say what the tolerances are . The approach you use determines that level and the inspection procedure as well.
        The most accurate machining requirement in the world is making a telescope mirror ...and manual hands do a better job
        Rich
        Green Bay, WI

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        • #34
          You know, over 15 inches, and depending on the finish and accuracy requirement.

          I would consider starting with a piece of 2X3/4 rolled stock. Bending it to approximate radius. (.4 inches?) (through rollers if possible)

          Then clamping it to the mill table and running xy steps ..in .050 increments. With a second pas of .025 increments.
          The first pass is "only" a few hundred movements. With a DRO, it would go fast. Use a spread sheet to make a chart of positions.

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          • #35
            Read the freaking thread, no need to bend anything, no need to X and Y it , when you can swing it past the cutter.
            just use a piece of the thickness required x 4 inch by around 17 inch or 2 pces
            we don't know the tolerance, but if it is +_ 1/16... it could easily be done by swinging a router or circular saw , on a pivot bracket, even made of wood.. without a milling machine even.
            really guys , you are going out of your way to make this harder. No wonder its not already done..
            Last edited by 754; 09-15-2020, 03:43 AM.

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            • #36
              Just use the DRO, step drill with an end mill. Set the max cut to .050 or finer if you want it to be finer. You'll be done with it in 2 hours, no additional jigs or setup needed. I've been using the smooth arc function a lot lately. Otherwise I am going to tell you to stop fighting the battle and get a CNC.

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              • #37
                This is the principle that tappet grinders work on, but I am under the impression that they actually grind a part of an ellipse rather than a true constant-radius curve. I'll have to think about it some more.

                Originally posted by Denny98501 View Post
                Long radii are actually a lot easier and a whole lot faster if you don't have CNC.
                Analog is the way to go for a wonderfully smooth curve instead of a thousand tiny little steps.
                You need a manual milling machine, a fly cutter and small rotary turntable.
                Below is a photo of a 200 inch radius of curvature polishing tool that we made.
                Below that is a reference showing how mirror makers produce long radius curves.
                After 15 minutes of setup, it took about 10 minutes to rotate the turntable and produce the curve.
                The turntable is tilted the amount of the Sag or calculated as shown below.
                If tilted the other direction, a concave curve will be made.
                If you only have a small fly cutter, the curve can be generated in two passes, but the angle will have to be recalculated.
                Hope this helps,



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                • #38
                  Originally posted by CalM View Post
                  Does that imply a numerical control on the laser/waterjet?
                  Yes, I suppose it does
                  Originally posted by CalM View Post
                  The OP mentioned WITHOUT CNC!
                  The subject line mentions milling, so I read Dan's post as how to do it without a CNC mill
                  Originally posted by CalM View Post
                  This could be best sumerized as "farm it out" but where is the pleasure in that?
                  The pleasure is getting the part with none of the work!
                  Seriously, I have had parts laser cut for less than I could personally purchase the material. It helps to supply the cutter with an actual drawing + DXF pattern instead of a napkin sketch though.

                  Location: Northern WI

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                  • #39
                    I thought this was a hobby machinist forum. You know, doing stuff for the pleasure in the doing.

                    I don't think a 1/2 inch depth of cut into SS is going to be router work pivoting off a 5 foot compass..

                    Abrasive machining maybe... swap the router for a portable belt sander.. Take it easy, It will be a while till it's done.

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                    • #40
                      Ooh jeepers, missed the SS part., I apologize....was concentrating more on the technical work holding , tool guiding aspect..
                      to clear up. Then use of compass is for layout, actual swinging fixture would be more like a motorcycle swingarm, much more rigid.

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                      • #41
                        Originally posted by CalM View Post
                        I thought this was a hobby machinist forum. You know, doing stuff for the pleasure in the doing.
                        I never got that impression. It says home shop machinist, not hobby machinist.

                        *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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                        • #42
                          Draw the arc in CAD, draw a line at either 0, 90, 180, or 270. Now draw a line at both the start and ending circle centers. Get your angular start and stops and go program the Smooth Arc. OR, in the CAD package, record all of your X and Y movements every .050" and do it the really old fashioned way.

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                          • #43
                            Final use dictates process needed. Are the radius' riding on something (i.e a trunion for a scope?) or are they simply a bracket for something and just needs to look nice and uniform? If the former, a long pivot could be setup with some tubing, or plywood etc to make an accuarate jig. If the latter, print it out on paper and transfer to material. Cut out with a bandsaw/jigsaw/plasma, and sand/file to shape. You could alternatively figure out a bunch of "plunge points" and just step it off then file to fit. Goes quicker than expected, although I haven't had to do that/generate points in many years.

                            You don't need a full fledged engineering print, just the end use that will help drive the decision making process.

                            I get roped into a lot of projects like this, and sometimes it's frustrating trying to get info out of people as to the end use. They don't think it's important, but it's often the most important detail that dictates whether the item will actually work or not. Just tell me what it's for and I can come up with the required clearances, and tolerances that will produce a workable part.

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                            • #44
                              Originally posted by Dan Dubeau View Post
                              I get roped into a lot of projects like this, and sometimes it's frustrating trying to get info out of people as to the end use. They don't think it's important, but it's often the most important detail that dictates whether the item will actually work or not. Just tell me what it's for and I can come up with the required clearances, and tolerances that will produce a workable part.
                              Just tell them, "I'm sure you can see the pretty picture in your head, but I can't see the pretty picture in your head." I get people all the time who want custom work done; but refuse to take the time to give me any details. Sometimes not even good working generalities. Grudging one incomplete sentence phrases typed with their thumbs on their cell phone seem to be the norm for some folks. I'm not exaggerating. I have to deal with it atleast a couple times every week.

                              *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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                              • #45
                                The less info you give out, the more it may cost
                                I notice on this one.. still no tolerance or end use stated. .. wtf.. they expect us to figure it all out ?
                                help us ..to help you. .

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